Frosting won't thicken
I'm making a frosting for red velvet cupcakes. The recipe calls for you to heat 1 c milk and 1/4 c flour together until thickened. You then cream 1 c sugar with 1 stick butter and 1/2 c shortening. Then you add the flour mix and 1 tsp. vanilla extract to the creamed butter/shortening and mix on high until it reaches spreading consistency. But my icing is more like soup. Tasty soup, but it's still a problem. Any idea if I can save this icing by thickening it up somehow?